1. Field of the Invention
The present invention relates to a small-size quartz-crystal oscillator designed for easy incorporation into integrated circuits in electronic devices, and more particularly to a crystal oscillator for surface mount applications which has an oscillation frequency variable in an increased range.
2. Description of the Related Art
Surface-mount crystal oscillators have a casing suitable for use in surface mount applications, and a quartz-crystal unit and an integrated circuit (IC) chip are sealed in the casing, the IC chip comprising an oscillating circuit using the crystal unit. Since such surface-mount crystal oscillators are small in size and weight, they are widely used as a frequency and time reference source in portable electronic devices such as cellular phone apparatuses. In recent years, surface-mount crystal oscillators are required to have a smaller size, e.g., a planar profile of 4 mm×2.5 mm or less. To meet such a requirement, the crystal blanks used in the surface-mount crystal oscillators are reduced in size and hence the variable range of oscillation frequencies thereof is also reduced. There are demands for improving the reduced variable range of oscillation frequencies.
FIG. 1 shows a cross-sectional structure of a conventional surface-mount crystal oscillator. As shown in FIG. 1, the conventional surface-mount crystal oscillator has casing 1 substantially in the form of a rectangular parallelepiped which has a recess defined in a principal surface thereof and having a step. The conventional surface-mount crystal oscillator is fabricated by fixing IC chip 2 to the bottom of the recess in casing 1 according to face-down bonding, fixing opposite sides of one end of crystal blank 3, which constitutes a crystal unit, to the step with conductive adhesive 4, and thereafter placing cover 5 on casing 1 to seal the recess. Crystal blank 3 and IC chip 2 are electrically connected to each other by a conductive path formed on an inner surface of the recess in casing 1. Connecting electrodes for external connection are formed on an outer surface of casing 1, and are electrically connected to IC chip 2 by a conductive path which extends through casing 1.
IC chip 2 comprises an integrated circuit except quartz-crystal unit 3A (crystal blank 3) of the crystal oscillating circuit, and may include a temperature compensating circuit, for example, in addition to basic oscillating circuit components. As shown in FIG. 2, the oscillating circuit comprises capacitors 6a, 6b connected to respective opposite ends of crystal unit 3A and ground to a reference potential, or a ground potential in FIG. 2, thus providing a resonance circuit. A C-MOS (Complementary Metal Oxide Semiconductor) inverting amplifier (inverter) 7 is connected parallel to crystal unit 3A for amplifying a resonance frequency component of the resonance circuit through a feedback loop. Feedback resistor 8 is connected across inverter 7. An oscillation output Vo is produced from an output terminal of inverter 7.
The oscillation frequency of the crystal oscillator generally depends on the resonance frequency of the resonance circuit, and is strictly determined by a equivalent series capacitance (so-called load capacitance) including inverter 7 as viewed from crystal unit 3A. The oscillation frequency is specifically a series resonance frequency determined by an inductive component provided by crystal unit 3A and a circuit capacitive component mainly provided by oscillating capacitors 6a, 6b. 
With the above surface-mount crystal oscillator, if the size of crystal blank 3 (crystal unit 3A) is smaller in size, then the variable range of oscillating frequencies is reduced. Therefore, when oscillators of various different oscillating frequencies are to be fabricated, since the range of oscillating frequencies that can be generated by one crystal unit 3A (crystal blank 3) is narrow, it is necessary to provide a large number of different crystal blanks 3, resulting in a reduction in productivity.
Specifically, a variable range of oscillating frequency f is represented by a frequency deviation Δf/fs which is the difference Δf (=f−fs) between the oscillating frequency f and a series resonance frequency fs of the crystal unit, as divided by the series resonance frequency fs, and expressed by the following equation (1):Δf/fs=C½(C0+CL)  (1)where C0 represent a parallel equivalent capacitance (i.e., inter-electrode capacitance) of crystal blank 3, C1 a series equivalent capacitance, and CL a load capacitance.
Consequently, if the size (plate area) of crystal blank 3 is reduced, the parallel equivalent capacitance C0 remains substantially unchanged as it needs a certain electrode area, and the series equivalent capacitance C1 is decreased. Therefore, the frequency deviation Δf/fs is also reduced, and the variable oscillation frequency range is narrowed. Thus, many different crystal blanks 3 need to be available in order to produce crystal oscillators having oscillating frequencies which meet demands in the market.